It is well known that the power consumption for any given electric motor, during acceleration, and especially during acceleration from zero, is much greater than the power consumption required for steady state operation. The operation of such electric motors under conditions requiring repeated start-stop functions therefore involves a somewhat excessive power consumption.
Numerous attempts have been made to reduce this excessive power consumption during start-up, or acceleration from zero. One approach has been to provide an electric motor having an armature with multiple windings which may be connected or disconnected at various speeds.
Other systems employ two or more separate windings, but with only limited success. Another approach altogether has been to provide for a two-speed operation, for example in elevator and hoist motors, where a slow and fast operation is required, by providing what are in effect two separate electric motors, with armatures actually fastened to the same shaft, in tandem.
However, such systems are of somewhat limited utility, and only provide for a very limited improvement over the use of a single motor.
An improved system is described in U.S. Pat. No. 4,525,655 entitled "Two Stage Electric Drive", invented by David E. Walker.
In this system two electric motors of different horsepower ratings are connected through an epicyclic gear train. A one-way rotational restraining device is placed on the portion of the drive train connected to the larger motor. The system operates, under acceleration, with the smaller motor starting first, and through a reducing drive in the epicyclic gear train, applies power to a drive shaft. Once a predetermined speed is reached, the second motor is started up, and the two motors are eventually operated at the same speed, providing a direct one-to-one drive to the drive shaft. During the first acceleration phase when the smaller motor is operating, the epicyclic gear train and the second motor are prevented from moving in the reverse direction by means of the one-way restraining device.
This system produces considerable advantages, in reducing the peak power consumption during acceleration which is normally experienced with electrical motors. Even with this improved system however there were still peaks occurring about the time the second or larger motor was started up, although such peaks were not as high as would have been the case in an ordinary electric motor.
Other problems arise in the use and operation of electrical motors, particularly large electrical motors. It is well known that electrical motors operate at maximum efficiency at their highest designed speed. They also operate most efficiently when the load applied to the motor is a steady state load. However, it frequently occurs that large electric motors are used in situations where the load is variable or intermittent. In these cases, there is considerable difficulty in controlling the electrical motor, when the load drops, and is considerable excess power consumption as a result. Complex controls are used to overcome this problem.
It is clearly desirable from a viewpoint of economy, and flexibility to provide an electric drive system which is capable of providing smooth continuous acceleration from zero, up to a desired maximum, and in which peaks in power consumption, are minimized, and at the same time providing such advantages within a reasonable cost.
Furthermore, it is clearly desirable to provide an electric drive system which is capable of operating under variable load conditions, with a high degree of efficiency, and with a reduced power consumption as compared with conventional electric motors.